The combination of the two-photon line scanning apparatus and the fluorescence correlation spectroscopy (FCS) apparatus can in principle improve the averaging time required in an FCS experiment by more than two orders of magnitude, by collecting and recording fluorescence fluctuations from an array of independent probe volumes. Alternatively, it yields a simultaneous determination of the spatial variation of the absolute concentration and diffusion constant along the focused line, with sub-micron resolution. Efforts are currently underway to determine the diffusion constant inside fluorescently labeled living RBL cells using this technique.